The chiral fluoroquinolone known under the name S-(−)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1H,5H-benzo[i,j]quinolizine-2-carboxylic acid is described in JP Patent 63,192,753A, JP Patent 05,339,238A, and in our pending U.S. patent applications Ser. Nos. 09/566,875, 09/640,947 and 09/802,793 and WO 00/68229, PCT Application Nos. PCT/IN00/00111 and PCT/IN01/00097.
S-(−)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1H,5H-benzo[i,j]quinolizine-2-carboxylic acid is an optically active isomer of the racemic compound which is claimed in U.S. Pat. No. 4,399,134.
S-(−)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1H,5H-benzo[i,j]quinolizine-2-carboxylic acid has an aqueous solubility of 0.8–2.0 mg/ml over the pH range 8.0–9.5 at 28° C., thus creating problems in having to formulate the drug as a tablet or capsule, or in making formulations for gavage and parenteral injection. The need for a salt is clearly indicated, as the lack of an appropriate salt form can hinder the development of dosage forms acceptable for systemic use in mammals.
S-(−)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1H,5H-benzo[i,j]quinolizine-2-carboxylic acid has a pKa value of 6.80 suggesting a weak acid character and thus an ability to form a salt with an appropriate base. Generally, conversion of a pharmacologically active compound into a salt form induces a change in the compound's physicochemical properties such as solubility, absorption velocity, etc.
Pharmaceutically more desirable salt forms may be selected by studying whether or not a crystalline or amorphous form, or polymorph or pseudopolymorph can be produced, and determining the properties including its physicochemical or biological properties. A pseudopolymorph is a polymorph that differs from a true polymorph by the incorporation of solvent (Solid-state Chemistry of Drugs, 2nd Ed. S. R. Byrn et al (Eds). SSCI, Inc. 1999, p-514).
Pharmaceutically acceptable salts of racemic 9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1H,5H-benzo[i,j]quinolizine-2-carboxylic acid such as salts with inorganic bases and organic bases are mentioned in the text of Otsuka's U.S. Pat. No. 4,399,134. Besides salts with inorganic bases and organic bases, amino acid salts of S-(−)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1H,5H-benzo[i,j]quinolizine-2-carboxylic acid are identified in our pending U.S. patent application Ser. No. 09/566,875 and U.S. patent application Ser. No. 09/640,947, WO 00/68229 and PCT Application No. PCT/IN00/00111. Arginine salt forms of S-(−)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1H,5H-benzo[i,j]quinolizine-2-carboxylic acid are identified in our pending U.S. patent application Ser. No. 09/802,793 and PCT Application No. PCT/IN01/00097. The subject matter of these applications is incorporated herein by reference. To date, no literature reference teaches about the amino acid salts of RS-(+/−)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1H,5H-benzo[i,j]quinolizine-2-carboxylic acid or R-(+)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1H,5H-benzo[i,j]quinolizine-2-carboxylic acid.
S-(−)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1H,5H-benzo[i,j]quinolizine-2-carboxylic acid, L-arginine salt 0.25 hydrate and S-(−)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperin-1-yl)-5-methyl-1-oxo-1H,5H-benzo[i,j]quinolizine-2-carboxylic acid, L-arginine salt, 0.75 hydrate described in Examples 7 and 8 of U.S. patent application Ser. Nos. 09/566,875 and 09/640,947, WO 00/68229 and PCT Application No. PCT/IN00/00111 respectively are highly hygroscopic and turn into syrups on exposure at a relative humidity of 41%. Additionally, arginine salt forms described in our pending U.S. patent application Ser. No. 09/802,793 and PCT Application No. PCT/IN01/00097 are either amorphous or only partially crystalline and are to a certain degree hygroscopic. Many hydrates and salts associated with water are susceptible to changes in humidity, are hygroscopic under adverse storage conditions and during pharmaceutical processing of them to medicament forms.
The amino acid salts, inorganic base and alkali salts and organic base salts and specially the arginine salt forms of S-(−)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1h,5H-benzo[i,j]quinolizine-2-carboxylic acid were prepared and studied by the inventors and it was found that:
(a) an arginine salt may exist in a crystalline form having distinctive physicochemical, solubility and stability properties;
(b) the crystalline arginine salt is less prone than the sodium salt to absorb moisture at specified humidity levels;
(c) the crystalline arginine salt, possesses a lower propensity to cause phlebitis than the sodium and potassium salts as determined in rats by intravenous administration; and
(d) the crystalline arginine salt is less toxic in rodents than the alkali salt forms.
In summary, the crystalline form of the arginine salt of S-(−)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1H,5H-benzo[i,j]quinolizine-2-carboxylic acid has been found by the inventors to have very desirable properties in possessing under specified conditions, less hygroscopicity, favourable aqueous solubility, a low propensity to cause phlebitis, and favourable acute toxicity values. This form is expected to be very useful as a pharmaceutical agent as compared with the previously described arginine salt forms, sodium salt, other inorganic base/alkali salts, organic base salts and other amino acid salts. These advantages will be apparent from the experimental data shown hereafter.